Calcium sulphate-based products are widely used in the construction of buildings, for example, to form internal partitions (using wallboard, also known as dry wall, gypsum board or plaster board) and ceilings or to encase ducts (e.g. ventilation ducts) within buildings.
Calcium sulphate-based products such as wallboard are typically formed by drying an aqueous slurry of the hemihydrate of calcium sulphate (CaSO4.½H2O), also known as calcined gypsum or stucco, between two sheets of lining paper or fibreglass matting. As the slurry dries and the calcined gypsum is hydrated, a hard, rigid core of gypsum (calcium sulphate dihydrate—(CaSO4.2H2O)) sandwiched between the lining sheets/mats is formed.
When wallboard is exposed to high temperatures such as those experienced in a building fire, or those experienced by wallboards used for encasing ducts carrying high temperature fluids, the water of crystallization contained within the gypsum is driven off to yield the anhydrite of calcium sulphate. Initially, this has the advantage that heat transfer across the wallboard is reduced thus helping to contain the heat emanating from a duct or generated during a building fire. However, at temperatures around 400-450° C., the initially formed AIII phase anhydrite (also known as γ-CaSO4 or “soluble” anhydrite) converts to the AII phase (or “insoluble” anhydrite) and this phase change results in shrinkage of the wallboard i.e. a loss of dimensional stability. This shrinkage (which may be around 2% of the wallboard's length or width or around 6 vol %) often causes the wallboards to pull away from their supporting structures. This is obviously undesirable. In situations where wallboard is used for internal partitions and a fire breaks out, shrinkage can leaves gaps exposing rooms adjacent to the fire source to the effects of the heat/fire. Gaps also allow ingress of oxygen into the fire source thus fuelling the fire and negating the effects of any fire doors.
At higher temperatures (in excess of 600° C.), the insoluble anhydrite goes on to sinter resulting in large reductions in wallboard volume. This results in extreme shrinkage which eventually causes collapse of the internal walls/ceilings/duct casings as they are no longer held by their supporting structures.
Furthermore, once the chemical composition of the gypsum has been altered by the heat, the wallboard loses strength and, ultimately, structural integrity. Typically, the gypsum core of wallboard that has been exposed to high temperatures such as those generated during building fires crumbles to a fine dust and thus the wallboard effectively disintegrates.
Efforts have been made to improve the fire resistance of calcium sulphate-based products in an attempt to reduce shrinkage and/or maintain strength/structural integrity.
It is known e.g. from U.S. Pat. Nos. 2,526,066 and 2,744,022, to add a combination of unexpanded vermiculite and non-combustible fibres to the aqueous calcined gypsum slurry during the manufacture of wallboard.
During heat exposure the vermiculite contained within the wallboard core expands by an amount comparable to the amount of gypsum shrinkage thus resisting the shrinkage of the wallboard. The fibres, which are known to be asbestos and/or glass, form a network which mechanically bind the gypsum core together and reduces the likelihood of mechanical failure.
Wallboard containing unexpanded vermiculite and/or glass fibres has found extensive commercial excess.
U.S. Pat. No. 3,616,173 proposed adding small amounts (preferably about 2-5 wt %) of clay, colloidal silica or colloidal alumina to the gypsum core in addition to the glass fibres and vermiculite.
The intention was to reduce the density of the fire resistant wallboard. Amounts greater than 20 wt % were found to result in a weak core that did not bind satisfactorily with the paper lining sheets.
US2003/0138614 discloses a fire resistant gypsum wallboard containing, in addition to unexpended vermiculite and glass fibres, 3-25 wt % of a mineral additive which may be a clay and 3-15 wt % hydrated alumina. Best results are achieved using 10-15 wt % of a clay which comprises 25% kaolinite.
U.S. Pat. No. 4,664,707 discloses a gypsum wall board made from a slurry containing glass fibres, calcium sulphate crystal fibres and 0.5-5 wt % clay. The clay is preferably a kaolinitic clay.
U.S. Pat. No. 6,569,541 discloses a water-resistant gypsum wallboard containing 5-15 wt % of a mineral additive which may be a clay such as kaolinite.
U.S. Pat. No. 5,985,013 discloses an ablative type heat protecting material containing calcium sulphate hemihydrate and a hydrated salt. A number of hydrated salts are used including magnesium nitrate hexahydrate (used in an amount of 40 wt % based on the weight of dry ingredients). The time taken for heat transfer across the heat ablative material was recorded. No mention is made of any effect on the shrinkage of the material after heating.
Calcium sulphate-based products are also used to cast metal or glass objects. Calcium sulphate moulds are heated to 700-900° C. prior to being filled with molten metal/glass. It is important to control high temperature shrinkage of such calcium sulphate-based moulds to ensure that the moulds do not leak and to ensure that the cast metal/glass products are not warped.
A preferred aim of the present invention is to provide an improved fire/heat resistant calcium-sulphate-based product having improved strength, hardness and structural integrity after heat exposure e.g. during a building fire. Such an improved fire resistant product may have particular use as a building product e.g. wallboard or panels for forming internal partitions in buildings, ceiling tiles, wallboard or panels for encasing ventilation/smoke extraction ducting, joint filler materials for joining wallboard/panels/tiles or for moulds for use in metal/glass product casting.